1. Field of the Invention
The present invention relates to a rubber-steel cord composite and a pneumatic tire for passenger cars. More particularly, it relates to a rubber-steel cord composite showing excellent adhesion at high temperatures and excellent durability, and a pneumatic tire for passenger cars showing excellent durability without any adverse effect on performance in an inflated condition in which pressure inside the tire (hereinafter referred to as internal pressure) is maintained and which can be safely used under decreased internal pressure.
2. Description of the Related Art
Composites having steel cords embedded in a rubber composition are used in tires, belts and hoses. Stable adhesion between the steel cord and the rubber composition is required to increase durability. To achieve stable adhesion between the steel cord and the rubber composition, heretofore, direct adhesion in which the cord is plated with brass, i.e., an alloy of copper and zinc, and the brass-plated cord is reacted with sulfur in the rubber composition is generally conducted.
On the other hand, various attempts have been made to achieve stable adhesion by adopting a suitable structure of a steel cord.
For example, the decrease in the life of products due to corrosion of steel filaments by water that penetrates into the products has been a problem in products reinforced with steel cords. When a steel cord has a cavity, water penetrating into the steel cord through cracks formed in a product is transferred through the cavity to other portions of the steel cord in the longitudinal direction of the cord. As the result, the formation of rust due to water spreads to a wider area and adhesion of the rubber and the steel cord decreases at portions having rust. Finally, separation takes place.
To prevent such expansion of corrosion into a wider area, cord structures that allow penetration of rubber sufficiently into spaces inside the cord (spaces between metal filaments) through gaps between adjacent filaments when vulcanization is conducted under a high pressure, have been proposed.
As an examples of the above cord structure, a so-called 1+5 structure comprising one core filament and 5 sheath filaments, which contains gaps between the sheath filaments to facilitate penetration of rubber and can be produced with high productivity because the structure can be formed in a single twisting step, is disclosed in Japanese Patent Application Laid-Open No. (hereinafter abbreviated as JP-A No.) 60-38208 and JP-A No. 59-1790.
However, although the average space of the gaps is sufficient, the above structure has a drawback in that the sheath filaments are not uniformly arranged and may contain portions where filaments are tightly attached together. Therefore, portions not penetrated by rubber may be formed due to random variations of the structure of the cord in the production of the cord.
It is suggested in JP-A 56-131404 that a cord having a 1+5 structure may be formed using a core filament made to have a slightly wavy shape. However, because the diameter of the core filament is smaller than the diameter of the sheath filaments, the above structure has drawbacks in that gaps between sheath filaments are small to make the penetration of rubber difficult, that the effect obtained by the wavy shape decreases due to decreased rigidity of the core filament and that the strength decreases when the core filament is shaped to a larger degree to improve penetration of rubber.
A structure can also be considered wherein the diameter of the core filament is made larger than the diameter of the sheath filament and the gaps between adjacent sheath filaments are kept to a specific size or larger to achieve penetration of rubber into the inside of the cord. However, when this structure is used, the weight of the entire cord increases and productivity decreases. Penetration of rubber is insufficient because deviations occur in the disposal of portions of the sheath filaments, which may become attached together. Therefore, expansion of corrosion into a wider area cannot be prevented sufficiently.
As pneumatic tires that can be used under reduced internal pressure, i.e., under a so-called run-flat condition, (hereinafter referred to as pneumatic safety tires), two types of tires are known with respect to the tire wheel. One such tire is a tire of the internal wheel type, in which an internal ring wheel made of a metal or a synthetic resin is attached to a rim at a portion inside the air chamber of the tire. The other such tire is a tire of the side reinforcement type, in which a layer of a relatively rigid rubber composition is disposed in the vicinity of a carcass in an area ranging from the bead portion to the shoulder portion of the tire side wall. Of these two types, the tire of the internal wheel type has a higher ability to support a load in the run-flat condition and is used for vehicles for transportation of goods and vehicles for military use which do not require a high degree of riding comfort. The tire of the side reinforcement type is used for vehicles carrying a relatively small load and requiring a higher degree of riding comfort. Both types of tires have been accepted as tires showing useful performance.
The tire of the side reinforcement type which is suitable for a pneumatic tire for passenger cars is reinforced with a relatively rigid reinforcing rubber layer having a cross-section of a crescent shape which is disposed on an inner face of the carcass layer in the side wall portion such that one end portion thereof is placed at a position under a belt layer, with the carcass disposed between them, and the other end portion is disposed at a position over a rubber filler. When the tire has a puncture and the air inside is lost, the load is carried by the rigidity of the side wall itself which is reinforced with the reinforcing rubber layer and the tire can be used for a predetermined distance in the run-flat condition although the speed must be decreased to some degree.
However, it is the actual situation at present that, even when the tire of the side reinforcement type is used, performance of the tire is not satisfactory in the run-flat condition.
Although the load carried by a general use tire for passenger cars is relatively small, the load per one tire is as large as about 500 kgf in the case of large size passenger cars. In this case, bending of side walls increases in a punctured condition and the side walls completely buckle under a dynamic load, which increases several times during driving The punctured tire is used while this phenomenon repeatedly takes place in the tire. As the result, the bead portion in the side wall is pushed up by a flange of the rim The cover rubber and the turned-up portion of the carcass which are placed between the curved flange and the rubber filler melt by heat or are fractured. The tire in this condition cannot be used any more even after the portion of the puncture has been repaired.
When a safety tire of a size for general purposes is driven in the run-flat condition, the temperature inside the tire increases to a temperature as high as 200xc2x0 C. or higher. Therefore, adhesion between rubber and fiber at a high temperature is not sufficient even when fibers having excellent heat resistance such as polyethylene terephthalate (PET) is used. Specifically in the case of a tire using PET for the carcass, the main cause of trouble during use in the run-flat condition is separation at the interface of PET and an adhesive layer. Moreover, even when a fiber showing excellent adhesion with rubber at high temperatures such as 66-Nylon is used, the surface of the cord is softened and the reinforcing effect decreases in a tire at a high temperature. Specifically in the case of a tire using 66-Nylon for the carcass, the main cause of trouble during use in the run-flat condition is separation due to melting of the surface layer of the 66-Nylon cords. Therefore, further improvement in durability in the run-flat condition is desired in both the case in which PET is used for the carcass and the case in which 66-Nylon is used for the carcass.
As the method for improving durability of the tire of the side reinforcement type in the run-flat condition, bending may be suppressed by increasing the thickness of the reinforcing rubber layer and the thickness of the bead filler to decrease the generated heat, or bending may be suppressed by increasing the number of layers in the carcass to decrease the generated heat. However, when these methods are used, the tire generally becomes more rigid and the weight of the tire increases. Therefore, performance in the inflated condition, such as rolling resistance and riding comfort during vibration, tends to become inferior and these methods are not preferable. In other words, excellent properties in the inflated condition are naturally required in combination with durability in the run-flat condition, and the above methods are not preferable.
An object of the present invention is to provide a rubber-steel cord composite showing excellent adhesion at high temperatures and excellent durability, and a pneumatic tire for passenger cars which shows no separation at the interfaces of adhesion during use, particularly during use in the run-flat condition, and exhibits excellent durability.
A steel cord constituting the rubber-steel cord composite in accordance with a first aspect of the present invention has, in a planar image of the steel cord formed by X-ray passing through the steel cord, a fraction R of the total area occupied by filaments of from 0.45 or more to 0.95 or less in an arbitrarily selected portion of the steel cord having a length of 15 mm in an axial direction of the cord,
wherein the length of 15 mm in an axial direction of the cord means a length of 15 mm in an axial direction of an actual cord and the fraction R of the total area occupied by the filaments is expressed as R=F/A, A representing the total area of the cord and F representing the area of the cord occupied by the filaments.
By specifying the fraction of the total area occupied by the filaments in the composite as described above, penetration of rubber into inner portions of the cord is facilitated and a rubber-steel cord composite exhibiting excellent durability without adverse effects on adhesion of the rubber and the steel cord even at high temperatures can be provided.
In the rubber-steel cord composite of the present invention, it is preferable that the filaments of the steel cords are present substantially independently of each other in a matrix rubber.
In the rubber-steel cord composite of the present invention, it is also preferable that the steel cords in the composite have a 1xc3x97n structure or a 1+n structure, wherein n is in the range of 2xe2x89xa6nxe2x89xa67.
In the rubber-steel cord composite of the present invention, it is also preferable that the filament of the steel cords in the composite has a diameter in the range of from 0.125 to 0.275 mm.
A pneumatic tire for passenger cars in accordance with a second aspect of the present invention comprises a rubber-steel cord composite comprising a steel cord and a matrix rubber, and in a planar image of the composite formed by X-rays passing through the composite which is disposed in the tire or is taken out of the tire, a fraction R of the total area occupied by filaments of from 0.45 or more to 0.95 or less in an arbitrarily selected portion of the steel cord having a length of 15 mm in the axial direction of the cord.
The length of 15 mm in the axial direction of the cord means a length of 15 mm in the axial direction of an actual cord and the fraction R of the total area occupied by the filaments is expressed as R=F/A, wherein A represents the total area of the cord and F represents the area of the cord occupied by the filaments.
The pneumatic tire for passenger cars of the present invention may comprise: a pair of right and left bead cores; a carcass portion formed in a toroidal shape with a carcass ply which comprises a layer in which a plurality of cords disposed substantially parallel to each other are embedded in a matrix rubber and which has both end portions turned up around the bead cores; a belt portion comprising a plurality of layers disposed at an exterior side of the carcass portion in a radial direction of the tire; a tread portion disposed at an exterior side of the belt portion in a radial direction of the tire; and a pair of side wall portions disposed at the right and left of the tread portion.
In the pneumatic tire for passenger cars of the present invention, it is preferable that filaments of the steel cords in the rubber-steel cord composite used in the tire are present substantially independently of each other in a matrix rubber.
In the pneumatic tire for passenger cars of the present invention, it is also preferable that the steel cords in the rubber-steel cord composite used in the tire have a 1xc3x97n structure or a 1+n structure, wherein n is in the range of 2xe2x89xa6nxe2x89xa67.
In the pneumatic tire for passenger cars of the present invention, it is also preferable that the filament of the steel cord in the rubber-steel cord composite used in the tire has a diameter in the range of from 0.125 to 0.275 mm.
In the pneumatic tire for passenger cars in accordance with third aspect of the present invention, the portion comprising the rubber-steel cord composite of the second aspect is carcass.
In the pneumatic tire for passenger cars in accordance with the forth aspect of the present invention, the rubber-steel cord composite described in the second aspect of the present invention is disposed in the side wall portion as an insert layer.
In the pneumatic tire for passenger cars of the present invention, it is preferable that the side wall portions are reinforced with a rubber reinforcing layer.
When an organic fiber cord is used as a carcass ply cord of the pneumatic tire of the present invention, it is preferable that the organic fiber has a melting point of 245xc2x0 C. or higher.
In the pneumatic tire for passenger cars of the present invention, it is preferable that the carcass ply comprises polyester cords and/or polyamide cords.
In one preferable embodiment of this pneumatic tire, the carcass comprises a turned-up ply and a down ply, both plies comprising a plurality of steel cords disposed substantially parallel to each other and a matrix rubber, and the down carcass ply is disposed between a side wall portion and an outer face of the turned-up ply, and either one of the turned-up ply and the down ply is a separate-type carcass ply which is separated into portions under the belt portion. Further, at least one layer of the turned-up carcass ply and the down carcass ply comprises the rubber-steel cord composite described in the second aspect of the invention.
The following advantages are obtained by reinforcing both of the turned-up carcass ply and the down carcass ply with steel cords and by separating either one of the carcass plies under the belt portion. An increase in the temperature of the tire is suppressed by suppressing bending during use, particularly during use in a run-flat condition. An increase in the weight of the tire due to an increase in the weight of the carcass having the steel cords is held to a minimum. Performance of the tire is maintained at a higher level under the inflated condition in comparison with that of safety tires having a conventional reinforcing rubber layer. The pneumatic tire exhibits excellent durability and excellent riding comfort.
In the third aspect of the present invention, the combination of the down carcass ply having steel cords and the turned-up carcass ply having steel cords, which is disposed at an inner side of the down carcass ply, exhibits the following effects. Bending of the tire is suppressed by the carcass comprising at least two layers of the carcass plies reinforced with steel cords, which are disposed at the side wall portion, due to the excellent tensile and bending rigidities of the layers. The decrease in the riding comfort due to the carcass comprising steel cords can be suppressed by separating one of the down carcass ply and the turned-up carcass ply into portions under the belt portion. The resistance to cuts caused by an outside source is also improved by using steel cords. Moreover, from the standpoint of the weight of the tire, the weight can be decreased in comparison with a combination of one layer of a down carcass ply having steel cords and one layer of a turned-up carcass ply having steel cords both in full length.
In the pneumatic tire in accordance with the third aspect of the present invention, it is preferable that either one of the turned-up carcass ply and the down carcass is separated into portions disposed under the belt portion with a distance of at least 20% of a width of the belt portion between the portions.
It is further preferable that the carcass comprises a layer comprising the rubber-steel cord composite described in the second aspect of the invention. The layer may be a down carcass ply or a so-called insert layer, disposed in the side wall portion.
The following advantages are obtained by using an organic fiber cord having a melting point of 245xc2x0 C. or higher for one layer of the turned-up carcass ply and by disposing the rubber-steel cord composite having the specific structure at the exterior side of the above carcass as the down carcass ply. An increase in the temperature of the tire is suppressed by suppressing bending during use, particularly during use in the run-flat condition. An increase in the weight of the tire is held to a minimum. Performance of the tire is maintained at a higher level in the inflated condition in comparison with that of safety tires having a conventional reinforcing rubber layer. The pneumatic tire exhibits excellent durability.
In the present invention, the combination of the down carcass ply having the steel cords and the turned-up carcass ply having the organic fiber cords which is disposed at the inner side of the down carcass ply exhibits the following effects. To suppress bending of the tire, the carcass which has the steel cords having excellent tensile rigidity plays a role at an outer layer and the organic fiber plays a role with respect to bending of an inner layer of the tire caused by bending of the tire. The resistance to cuts caused by an outside source is also improved by using the steel cords disposed in the outer layer. Moreover, from the standpoint of the weight of the tire, the weight can be decreased in comparison with a combination of one layer of a down carcass ply having the steel cords and one layer of a turned-up carcass ply having the organic fiber cords.
The following advantages are obtained by disposing the rubber-steel cord composite having the specific structure at the side wall portions. Adhesion between the rubber and the reinforcing cords in the pneumatic tire for passenger cars does not decrease during use, particularly during use in the run-flat condition. Durability is further improved. Thus, the pneumatic tire for passenger cars exhibiting excellent durability can be provided without adverse effects on performance of the tire in the inflated condition.
The upper end portion of the rubber-steel cord composite may be disposed between an outer end portion of the belt portion and a central portion of the tread portion.
Preferably, the upper end portion of the rubber-steel cord composite covers 10% or more of an area between the outer end portion of the belt portion and the central portion of the tread portion.
The lower end portion of the rubber-steel cord composite may be disposed at a position lower than an upper end portion of a bead filler.
The rubber-steel cord composite may be disposed in such a manner that the lower end portion of the rubber-steel cord composite is disposed at a position of a lower end portion of the bead filler and an upper end portion of the composite is disposed at a position higher than an upper end portion of the bead filler, and preferably, that the rubber-steel cord composite has a length of 110% or more of a height of the bead filler.
The distance between a lower end portion of the rubber-steel cord composite and the outer end portion of the belt portion may be 25 mm or more.